Device and method for material processing by means of laser radiation

ABSTRACT

In a device for material processing by laser radiation, including a source of laser radiation emitting pulsed laser radiation for interaction with the material; optics focusing the pulsed processing laser radiation to a center of interaction in the material and a scanning unit shifting the positions of the center of interaction within the material. Each processing laser pulse interacts with the material in a zone surrounding the center of interaction assigned to the laser pulse so that material is separated in the zones of interaction. A control unit controls the scanning unit and the source of laser radiation such that a cut surface is produced in the material by sequential arrangement of zones of interaction. The control unit controls the source of laser radiation and the scanning unit such that adjacent centers of interaction are located at a spatial distance a≦10 μm from each other.

RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication No. 60/726,887, filed Oct. 14, 2005, which is incorporatedherein in its entirety by reference.

FIELD OF THE INVENTION

The invention relates to a device for material processing by means oflaser radiation, said device comprising a source of laser radiationemitting pulsed laser radiation for interaction with the material;optics focusing the pulsed processing laser radiation to a center ofinteraction in the material; a scanning unit shifting the positions ofthe center of interaction within the material, wherein each processinglaser pulse interacts with the material in a zone surrounding the centerof interaction assigned to said laser pulse so that material isseparated in the zones of interaction; and a control unit which controlsthe scanning unit and the source of laser radiation such that a cutsurface is produced in the material by sequential arrangement of zonesof interaction.

BACKGROUND OF THE INVENTION

The invention further relates to a method of material processing bymeans of laser radiation, wherein pulsed processing laser radiation isgenerated, focused for interaction to centers of interaction in thematerial, and the positions of the centers of interaction in thematerial are shifted, wherein each processing laser pulse interacts withthe material in a zone surrounding the center of interaction assigned tosaid laser pulse and material is separated in the zones of interactionand a cut surface is produced in the material by sequential arrangementof zones of interaction.

The invention further relates to a device for material processing bymeans of laser radiation, said device comprising a source of laserradiation emitting pulsed laser radiation for interaction with thematerial; optics focusing the pulsed processing laser radiation along anoptical axis to a center of interaction in the material; a scanning unitshifting the positions of the center of interaction within the material,wherein each processing laser pulse interacts with the material in azone surrounding the center of interaction assigned to said laser pulseso that material is separated in the zones of interaction; and a controlunit which controls the scanning unit and the source of laser radiationsuch that a cut surface is produced in the material by sequentialarrangement of zones of interaction.

The invention still further relates to a method of material processingby means of laser radiation, wherein pulsed processing laser radiationis generated and focused for interaction to centers of interaction inthe material along an optical axis, and the positions of the centers ofinteraction in the material are shifted, wherein each processing laserpulse interacts with the material in a zone surrounding the center ofinteraction assigned to said laser pulse, and material is separated inthe zones of interaction, and a cut surface is produced in the materialby sequential arrangement of zones of interaction.

These devices as well as corresponding methods of material processingare particularly suitable to produce curved cut surfaces within atransparent material. Curved cut surfaces are produced, for example, inlaser-surgical methods and, in particular, in ophthalmic operations. Indoing so, treatment laser radiation is focused into the tissue, i.e.below the surface of the tissue, to a center of interaction. Materiallayers in a surrounding zone of interaction are separated thereby. Thezone usually corresponds to the focus spot. The laser pulse energy isusually selected such that an optical breakthrough in the tissue formsin the zone of interaction.

In the tissue, a plurality of processes initiated by the laser radiationpulse take place in a time sequence after an optical breakthrough.First, the optical breakthrough generates a plasma bubble in thematerial. Once such plasma bubble has formed, it grows due to expandinggas. Next, the gas generated in the plasma bubble is absorbed by thesurrounding material and the bubble disappears again. However, thisprocess takes very much longer than the forming of the bubble itself. Ifa plasma is generated at a material interface which may even be locatedwithin a material structure, material removal is effected from saidinterface. This is then referred to as photoablation. In case of aplasma bubble separating previously connected material layers, oneusually speaks of photodisruption. For the sake of simplicity, all suchprocesses are summarized here by the term “interaction”, i.e. this termincludes not only the optical breakthrough, but also any othermaterial-separating effects.

For high precision of a laser-surgical method, it is indispensable toensure high localization of the effect of laser beams and to avoid, ifpossible, collateral damage to adjacent tissue. Therefore, it is commonin the prior art to apply the laser radiation in pulsed form so that thethreshold value for the energy density required to initiate an opticalbreakthrough is exceeded only in the individual pulses. In this respect,U.S. Pat. No. 5,984,916 clearly shows that the spatial extent of thezone of interaction substantially depends on the pulse duration only aslong as a pulse duration of 2 ps is exceeded. For values of few 100 fs,the size of the zone of interaction is almost independent of the pulseduration. Thus, high focusing of the laser beam in combination with veryshort pulses, i.e. below 1 ps, allows the zone of interaction to beinserted in a material with pinpoint accuracy.

The use of such pulsed laser radiation has recently become established,in particular, for laser-surgical correction of visual deficiencies inophthalmology. Visual deficiencies of the eye are often due to the factthat the refractive properties of the cornea and of the lens do notcause optimal focusing on the retina. This type of pulsing is also thesubject matter of the invention described herein.

The aforementioned U.S. Pat. No. 5,984,916 describes a method ofproducing a cut surface by suitably generating optical breakthroughs,thereby ultimately exerting a selective influence on the diffractiveproperties of the cornea. A multiplicity of optical breakthroughs aresequentially arranged such that the cut surface isolates a lens-shapedpartial volume within the cornea of the eye. The lens-shaped partialvolume separated from the remaining corneal tissue is then removed fromthe cornea via a laterally opening cut. The shape of the partial volumeis selected such that upon removal the shape and, thus, the refractiveproperties of the cornea are changed so as to cause the desiredcorrection of a visual deficiency. The cut surface required here iscurved and circumscribes the partial volume, thus necessitatingthree-dimensional shifting of the focus. Therefore, two-dimensionaldeflection of the laser radiation is combined with simultaneous shiftingof the focus in a third spatial direction. This is summarized here bythe terms “scanning”, “shifting” or “deflecting”.

When composing the cut surface by sequential arrangement of opticalbreakthroughs in the material, an optical breakthrough is generated manytimes faster than the time it takes until a plasma generated thereby isabsorbed by the tissue again. It is known from the publication of A.Heisterkamp et al., Der Ophthalmologe, 2001, 98:623-628, that, after anoptical breakthrough has been generated, a plasma bubble forms in theeye's cornea at the focal point where the optical breakthrough wasgenerated, which plasma bubble can grow together with adjacent bubblesto form macrobubbles. The publication explains that the joining of stillgrowing plasma bubbles reduces the quality of the cut. Therefore, saidpublication proposes a method wherein individual plasma bubbles are notgenerated immediately adjacent to each other. Instead, a gap is left ina spiral-shaped profile between sequentially generated opticalbreakthroughs, which gap is filled with optical breakthroughs and theresulting plasma bubbles in a second pass through the spiral. This isintended to prevent joining of adjacent plasma bubbles and to improvethe quality of the cut.

In order to achieve good quality of the cut, the prior art thus usesdefined sequences in which the optical breakthroughs are generated. Thisis intended to prevent joining of growing plasma bubbles. Since a cut isdesired, of course, wherein as few bridges as possible connect thematerial or the tissue, respectively, the plasma bubbles generatedultimately have to grow together in any case to form a cut surface.Otherwise, the material connections would remain and the cut would beincomplete.

Therefore, it is an object of the invention to generate good-qualitycuts in the material without having to observe defined sequences whenintroducing laser pulses.

According to the invention, this object is achieved in a first variantby a device of the first-mentioned generic type, wherein the controlunit controls the source of laser radiation and the scanning unit suchthat adjacent centers of interaction are located at a spatial distancea≦10 μm from each other. In the first variant, the object is furtherachieved by a method of the first-mentioned generic type, whereinadjacent centers of interaction are located at a spatial distance a≦10μm.

In a second variant of the invention, the object is achieved by a deviceof the first-mentioned generic type, wherein the fluence F of the pulsesfor each center of interaction is respectively below 5 J/cm². In thesecond variant, the object is also achieved by a method of thefirst-mentioned generic type, wherein the zones of interaction areexposed to pulses whose fluence F is respectively below 5 J/cm².

In a third variant of the invention, the object is achieved by a deviceof the second-mentioned generic type, wherein the control unit controlsthe source of laser radiation and the scanning unit such that the cutsurface comprises two portions located adjacent to each other along theoptical axis, and at least partially illuminates them with laser pulsesapplied within a time interval t≦5 s. Also in the third variant theobject is achieved by a method of the second-mentioned type, wherein thecut surface comprises two portions located adjacent to each other alongthe optical axis which are at least partially exposed to laser pulsesapplied within a time interval t≦5 s.

The invention is based on the finding that zones of interaction in thematerial influence each other. Thus, the effect of a laser beam pulsedepends on the extent to which previous laser exposures already tookplace in the vicinity of the center of interaction. From this, theinventors concluded that the pulse energy required to generate anoptical breakthrough or to cause material separation depends on thedistance from the nearest center of interaction. All of the variantsaccording to the invention take advantage of this finding.

The inventive minimization of the distance between centers ofinteraction, e.g. of the distance between the focus positions ofadjacent optical breakthroughs, according to variant 1 allows theprocessing pulse energy to be decreased. The parameter describing thepulse energy is the fluence, i. e. the energy per area or the arealdensity of energy. Thus, the inventive variant 1 with a distance of lessthan 10 μm addresses an aspect of the finding attributable for the firsttime to the inventors.

Another aspect is that the fluence of the processing laser pulses is nowsignificantly reduced. Thus, variant 2 relates to the same aspect asvariant 1, although it does not prescribe an upper limit for thedistance, but for the fluence.

Accordingly, all variants of the invention provide basic conditions forproducing a cut by introducing pulsed laser radiation, said basicconditions taking into consideration the effects of the immediatelyadjacent introduced pulse. Regarding the pulse length, the teaching ofU.S. Pat. No. 5,984,916 is applied here, i.e. pulses below 1 ps,preferably few 100 fs, e. g. 300-500 fs, are used. As far as theinvention defines an upper limit of the distance, this refers to thedistance from the closest center of interaction. Since a cut surface isusually produced by a multiplicity of sequentially arranged centers ofinteraction, the distance may be understood, for the sake of simplicity,also to be the mean value of the laser focus spacing for the laserpulses in the material. If the grating of centers of interaction whichis substantially two-dimensional along a cut surface is not symmetrical,distance can also be the characteristic mean spacing. It is known in theprior art to use a pulsed source of laser radiation and to modify someof the laser pulses emitted by said source such that they do not cause aprocessing effect in the material. Only some of the laser radiationpulses will then be used for processing. Whenever the presentdescription uses the term “laser radiation pulse”, “laser pulse” or“pulse”, this always means a processing laser pulse, i.e. a laserradiation pulse provided or formed or suitable for interaction with thematerial.

The complexity of equipment is reduced by the invention, because thepulse peak performance decreases. Due to the reduced distance of thecenters of interaction, the pulse repetition frequency increases if theprocessing duration is to be kept constant. Further, smaller plasmabubbles are produced in the case of optical breakthroughs, thus makingthe cut thinner. However, the prior art always worked with comparativelylarge distances between the centers of interaction and the fluence ofthe pulses was selected suitably high in order to securely obtainoptical breakthroughs and large plasma bubbles suitably adapted to thedistances.

At the same time, a lower fluence also reduces personnel hazards duringmaterial processing. This is of essential importance in ophthalmicmethods. It turns out to be particularly advantageous that it is nowpossible to work with lasers of hazard class 1M, whereas class 3 wasrequired in the prior art. This class required operating personnel, forexample a physician or a nurse, to wear protective goggles, whichnaturally makes patients feel uneasy. Such protective measures are nolonger necessary with the lasers of class 1M that are now possibleaccording to the invention.

Therefore, the invention also provides as a further embodiment, orindependently, a device for material processing by means of laserradiation, said device comprising an emitting source of laser radiationwhich emits pulsed laser radiation for interaction with the material,optics focusing the pulsed laser radiation to a center of interaction inthe material, a scanning unit shifting the position of the center ofinteraction in the material, wherein each processing laser pulseinteracts with the material in a zone surrounding the center ofinteraction assigned to said pulse, so that material is separated in thezones of interaction, and said device further comprising a control unitcontrolling the scanning unit and the source of laser radiation suchthat a cut surface is produced in the material by sequential arrangementof zones of interaction, wherein a laser of a hazard class below 3,preferably a laser of hazard class 1M, is employed. The indication ofthe hazard class relates to International Standard IEC 60825-1 in itsversion as effective Oct. 13, 2005. Analogously, there is provided(independently or as a further embodiment) a device for materialprocessing by means of laser radiation, said device comprising a sourceof laser radiation emitting pulsed laser radiation for interaction withthe material; optics focusing the pulsed laser radiation to a center ofinteraction in the material along an optical axis; a scanning unitshifting the position of the center of interaction in the material, eachlaser pulse interacting with the material in a zone surrounding thecenters of interaction assigned to said pulse and material beingseparated in the zones of interaction, said device further comprising acontrol unit controlling the scanning unit and the source of laserradiation such that a cut surface is produced in the material bysequential arrangement of zones of interaction, wherein a laser of ahazard class below 3, preferably a laser of hazard class 1M, is used.This is also useful as a further embodiment for each of theaforementioned devices or for each of the aforementioned methods,respectively. Unless explicitly indicated otherwise, this shall apply toeach described advantageous design, further embodiment or realization.

Tests carried out by the inventors have shown that an opticalbreakthrough sets in only above a defined threshold value M which is afunction of the distance a of adjacent centers of interaction accordingto the equation M=3.3 J/cm²−(2.4 J/cm²)/(1+(a/r²)²). An opticalbreakthrough is ensured for each individual laser pulse only at a pulsefluence above the threshold value M. The parameter r appearing in saidequation represents an experimentally recognized average range of theinfluence of adjacent zones of interaction. Depending on theapplication, there may be fluctuations here, so that a variation of thevalue between 3 and 10 μm is possible; preferably, r=5 μm.

In a further embodiment of the invention, the upper limit of pulsefluence mentioned for variant 2 of the invention will also be based onthe aforementioned dependence of the threshold value on the distance ofadjacent centers of interaction. Therefore, a farther embodiment ispreferred in which fluence exceeds the threshold value M by an excessiveenergy of no more than 3 J/cm². The range defined thereby provides aparticularly good quality of the cut, while initiation of an opticalbreakthrough is ensured at the same time. If the excessive energy werefurther increased, unnecessarily large plasma bubbles would be generatedand the quality of the cut would deteriorate.

However, producing a cut now no longer stringently requires working withoptical breakthroughs. The inventors have found that, if the zones ofinteraction overlap, material can be separated and, thus, a cut surfacecan be formed even at energies of the pulsed laser radiation below athreshold value for initiation of an optical breakthrough. Therefore, afurther embodiment is provided wherein the spatial distance a of thecenters of interaction of two sequential pulses is smaller than the sizeof the focus d, so that there is a mutual overlap of volumes of thematerial that are sequentially irradiated with laser radiation, i.e.zones of interaction. This embodiment results in material separationwithout formation of plasma bubbles, which leads to a particularlysmooth cut.

Advantageously, the fluence of the laser pulse can then also bedecreased below the already explained threshold value, because atissue-separating effect is still achieved due to overlapping of zonesof interaction. The individual laser pulse then no longer securelygenerates an optical breakthrough; the separation of tissue is causedonly if the zones of interaction overlap. This allows pulse energiesthat are orders of magnitude below those of the state of the art; at thesame time the quality of the cut is increased again, because zones ofinteraction, which are generated sequentially in time, overlap. Thus,the distance of the centers of interaction ranges from zero to thediameter of the focus, which is e.g. between 1 and 5 μm considering the1/e² diameter (e=Euler's constant).

Cutting according to the invention produces a very fine cut because, dueto the reduced distance or the reduced pulse energy, respectively,correspondingly small or even no plasma bubbles are worked with or canbe worked with. However, a fine cut surface can also be a disadvantage,e.g. if a surgeon wants to optically recognize at least part of the cutsurface. This is the case, for example, in laser surgery according tothe fs-LASIK method. The partial volume isolated therein by the actionof laser radiation, which volume is to be removed from the tissue by alateral cut, is usually freed first from any residual bridges to thesurrounding material by the surgeon using a spatula. For this purpose,the surgeon pushes the spatula into the pocket formed by the laterallyopening cut and traces the partial volume with the spatula. In case of avery fine, i.e. smooth cut surface, it may occur that the surgeon can nolonger see the profile of the cut surface in the material from outside.Therefore, he will not know where the periphery of the partial volumelies and will not be able to securely guide the spatula. In order tosolve these problems, a method of the above-mentioned type is providedwherein the cut surface is divided into at least two partial surfaces,and one partial surface is formed with operating parameters thatgenerate a coarser and, thus, rougher cut surface. In a device of theabove-mentioned type, the control unit carries out the correspondingcontrol of the laser source and of the scanning unit. Preferably, saidcoarser cut surface will be placed on the periphery, which is easilyrecognizable for the user and is of no importance to the quality of thecut surface, e.g. in ophthalmic surgery. Thus, the two partial surfacesdiffer from each other with respect to at least one parameterinfluencing the fineness of the cut surface. For instance, a possibleparameter is the fluence of the laser pulses used or the spatialdistance between the centers of interaction.

Combining this approach, which may be principally effected in differentways and is not restricted to the invention described herein, with oneof the aforementioned variants of the invention, it is convenient forthe control unit to control the source of laser radiation and thescanning unit such that the cut surface is composed of at least a firstand a second partial cut surface, the first partial cut surface beingproduced by controlling the source of laser radiation and the scanningunit according to one of the aforementioned inventive concepts, and thesecond partial cut surface being produced by controlling the source oflaser radiation so as to cause a pulse fluence of more than 3 J/cm²,preferably more than 5 J/cm². Of course, a>10 μm may be set then,because the plasma bubbles will be large. The latter partial surfacethen automatically has the desired coarser structure and facilitatesrecognition of the cut surface by the user or surgeon. The analogousmethod accordingly provides for the second partial cut surface to beproduced by a method of the invention at a pulse fluence of more than 3J/cm², preferably more than 5 J/cm².

Conveniently, the coarser partial surface will be selected such that itsurrounds the finer partial surface, so that the surgeon can clearlyrecognize the periphery of the cut surface and optical imaging at thetreated eye (in the case of ophthalmic surgery) is not adverselyaffected.

The finding upon which the invention is based further shows that thethreshold value required to securely achieve an optical breakthroughdecreases as the distance of the centers of interaction decreases.

The analysis carried out by the inventors further shows that the shapeof the plasma bubbles generated, which are formed as a result of theinteraction of the laser pulses with the material or the tissue,respectively, can be subject to a temporal change, as also indicated inthe publication by Heisterkamp et al. However, whereas this publicationfocuses on preventing a center of interaction from being located near ajust growing plasma bubble, it is now the object of variant 3 of theinvention that a deformation generated by a macrobubble will not affectthe quality of the cut. If a further optical breakthrough were placed ata defined position in deformed material or tissue, the position of thecenter of interaction within the material or tissue would be shifted assoon as said deformation is reduced by relaxation. Therefore, it isenvisaged according to the third variant to keep the time between theapplication of laser energy in two areas of the material or of thetissue, respectively, influencing each other so small that it is smallerthan a characteristic time for forming of macrobubbles. Said time isapproximately 5 s. Of course, this approach is required only if twoportions of the cut surface located adjacent to each other along theoptical axis are present, because only then can a deformation caused byproducing a cut surface portion have an effect on the formation of theother cut surface portion which is located adjacent thereto along theoptical axis.

This approach is particularly important in generating a partial volumeduring the fs-LASIK method. This partial volume, also referred to as alenticule, is generated by a posterior portion and an anterior portionof the cut surface, so that the cut surface as a whole circumscribes thelenticule. However, generating the posterior and anterior portionstogether within the characteristic time for forming the macrobubbles mayresult in relatively high demands on the scanning unit's speed ofdeflection or inevitably leads to special scanning paths. Preferably,this can be avoided by dividing the posterior and anterior portions intopartial surfaces and skillfully selecting the processing sequence ofthese partial surfaces.

In one embodiment, the two areas are subdivided into annular partialsurfaces. Since in the case of a lenticule the central partial surfacehas a much stronger influence on optical quality than the peripheralregions, first the cut corresponding to the central partial surface ofthe posterior portion and then that of the anterior portion is produced,so that the partial surfaces are formed immediately after each other.Then, the annular partial surface of the posterior portion, and that ofthe anterior portion is cut next. This principle can also be carried outwith as many partial surfaces as desired. Practical limits are given bythe fact that switching between the anterior and posterior portionsalways requires shifting of the laser focus along the optical axis,which for technical reasons takes up most of the time during scanning.

With this approach, it is important to note that the diameter of eachannular or circular posterior partial surface should be somewhat largerthan the diameter of the respective anterior partial surface generatednext. This ensures that the posterior partial cut to be produced nextmakes not only anteriorly located disruption bubbles acting as centersof scattering impossible. The minimum amount by which the posteriorpartial cut has to be larger than its associated anterior partial cut isgiven by the numerical aperture of the focusing optics.

A further way of pushing the time interval below the characteristic timeconsists in generating the posterior portion with a spiral of thecenters of interaction, said spiral extending from the outside to theinside, and in generating the anterior portion with a spiral extendingfrom the inside to the outside. This ensures that portions locatedadjacent to each other along the optical axis are formed at least in thecentral region within the 5 s time interval. Of course, this method canbe applied to the already mentioned divisions of partial surfaces.

It is therefore preferred that the control unit control the source oflaser radiation as well as the scanning unit such that at least some ofthe portions adjacent to the optical axis are illuminated immediatelysubsequent to each other in time by sequential arrangement of thecenters of interaction.

Analogous considerations also apply to the embodiment of the methodaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below, by way of exampleand with reference to the Figures, wherein:

FIG. 1 shows a laser surgical instrument for eye treatment;

FIG. 2 shows a diagram of the effect of laser radiation on the cornea ofthe eye for the instrument of FIG. 1;

FIG. 3 shows a schematic view illustrating how a partial volume isgenerated and isolated by the instrument of FIG. 1;

FIG. 4 shows a deflecting device of the instrument of FIG. 1;

FIG. 5 shows a block diagram illustrating the structure of theinstrument of FIG. 1;

FIG. 6 shows a relationship between the distance of the centers of theoptical breakthroughs generated by the instrument of FIG. 1 and thepulse energy, wherein possible operating ranges for the instruments ofFIG. 1 are illustrated;

FIG. 7 shows a representation similar to that of FIG. 6;

FIG. 8 shows a schematic top view of the eye's cornea for clearerillustration of the generated plasma bubbles' position or the cutsurface caused thereby, respectively;

FIG. 9 shows a sectional view of the representation of FIG. 8 along theline A1-A1;

FIG. 10 shows a schematic view illustrating the arrangement of aplurality of zones of interaction when producing the cut surface with aninstrument according to FIG. 1, and

FIGS. 11 and 12 show views similar to that of FIG. 10 for modified modesof operation.

DETAILED DESCRIPTION

FIG. 1 shows a laser surgical instrument for treatment of a patient'seye 1, said laser surgical instrument 2 serving to effect a refractivecorrection. For this purpose, the instrument 2 emits a treatment laserbeam 3 onto the eye of the patient 1 whose head is fixed in a headholder 4. The laser surgical instrument 2 is capable of generating apulsed laser beam 3 such that the method described in U.S. Pat. No.5,984,916 can be carried out. For example, the treatment laser beam 3consists of fs-laser pulses having a pulse repetition rate of between 10and 500 kHz. In the exemplary embodiment, the structural components ofthe instrument 2 are controlled by an integrated control unit.

As schematically shown in FIG. 2, the laser surgical instrument 2comprises a source of radiation S whose radiation is focused into thecornea 5 of the eye 1. Using the laser surgical instrument 2 a visualdeficiency of the patient's eye 1 is corrected by removing material fromthe cornea 5 such that the refractive properties of the cornea change toa desired extent. In doing so, said material is removed from the cornealstroma which is located below the epithelium and the Bowman membrane aswell as above the Decemet membrane and the endothelium.

Material removal is effected by separating material layers in the corneausing an adjustable telescope 6 to focus the high-energy pulsed laserbeam 3 to a focus 7 located in the cornea 5. Each pulse of the pulsedlaser radiation 3 generates an optical breakthrough in the tissue, suchoptical breakthrough in turn initiating a plasma bubble 8. Thus, thetissue layer separation covers a larger area than the focus 7 of thelaser radiation 3, although the conditions for achieving thebreakthrough are achieved only in the focus 7. Then, many plasma bubbles8 are generated by suitable deflection of the laser beam 3 duringtreatment. This is shown schematically in FIG. 3. The plasma bubblesthen form a cut surface 9 which circumscribes a partial volume T of thestroma, namely the material to be removed from the cornea 5. The cutsurface 9 is formed by sequential arrangement of the plasma bubbles 8 asa result of a continuous shift in the focus 7 of the pulsed laser beam3.

Due to the laser radiation 3 the laser surgical instrument 2 acts like asurgical knife directly separating material layers within the cornea 5without damaging the surface of the cornea 5. If a cut 16 is guided upto the surface of the cornea by further generation of plasma bubbles 8,material of the cornea 5 isolated by the cut surface 9 can be pulled outlaterally in the direction of the arrow 17 and can thus be removed.

On the one hand, displacement of the focus is then effected in theembodiment by means of the deflecting unit 10 shown schematically inFIG. 4, said deflecting unit 10 deflecting the laser beam 3, incident onan optical axis H of the eye 1, about two mutually orthogonal axes. Forthis purpose, the deflecting unit 10 uses a line mirror 11 as well as aframe mirror 12, which leads to two spatial axes of deflection locatedbehind each other. The point of intersection of the optical axis H andthe deflecting axis is then the respective point of deflection. On theother hand, the telescope 6 is suitably adjusted for focus displacement.This allows the focus 7 to be shifted along three orthogonal axes in thex/y/z coordinate system shown schematically in FIG. 4. The deflectingunit 10 shifts the focus in the x/y plane, with the line mirror allowingto shift the focus in the x direction and the frame mirror allowing ashift in the y direction. In contrast thereto, the telescope 6 acts onthe z coordinate of the focus 7. Thus, three-dimensional displacement ofthe focus 7 is achieved as a whole.

Due to the corneal curvature which is between 7 and 10 mm the partialvolume T also has to be curved accordingly. Thus, the corneal curvaturerequires a curved cutting plane. This is effected by suitable control ofthe deflecting unit 10 and of the telescope 6.

FIG. 5 shows a simplified block diagram of the laser surgical instrument2 for refractive surgery on the human eye 1. Only the most importantstructural components are shown: an fs laser serving as source ofradiation S, which laser consists of an fs oscillator V as well as ofone or more amplifying stages 13 and following which a compressor orpre-compressor 14 is arranged here as well; a laser pulse modulator 15on which laser radiation from the laser S is incident; the deflectingunit 10, realized as a scanner here; an objective for focusing into thetissue to be treated, said objective realizing the telescope 6, and thecontrol unit 17.

The laser S generates laser pulses having a duration in the fs range.First, the laser pulses reach the laser pulse modulator 15 whichinfluences the laser pulses (in a manner yet to be described) accordingto a control signal from the control unit 17. Next, at least thetreatment laser pulses reach the scanner 10 and pass through theobjective 6 into the patient's eye 1. There, they are focused andgenerate optical breakthroughs in the focus 7. The modulator sets theenergy of the laser pulses, i.e. the fluence of the individual laserpulses. As the modulator an AOM or an electro-optical modulator (EOM), aPockels cell, a liquid crystal element (LC element), a fiber-opticalswitching element or a variable attenuator, e.g. a neutral densityfilter, may be used.

The laser surgical instrument 1 can then work in different modes ofoperation which may each be realized separately or in combination andwhich relate to the energy or the fluence F of each laser pulse or tothe local distance at which the laser pulses are sequentially arrangedso as to generate the cut surface 9.

FIG. 6 shows a threshold value M as a graph illustrating therelationship between a spacing a at which the centers of interaction ofthe individual laser pulses are sequentially arranged within the eye'scornea 5 and the fluence F of each laser pulse. An optical breakthroughwith an ensuing plasma bubble is generated only at a fluence above thethreshold value.

The circles entered into the graph result from experimental measurementsand represent points of measurement. Measurement was effected at a pulseduration of 300 fs and a 3 μm spot diameter of the focus 7.

The instrument 1 may be operated in an operational range 18 according toFIG. 6 which may be defined by various boundary conditions. Thedifferent definitions correspond to different variants of the invention.All variants are based on the course of the threshold value M for thefluence F as a function of the distance a. This dependence isapproximated by the following formula: M=3.3 J/cm²−(2.4J/cm²)/(1+(a/r²)²), wherein r is a parameter representing the averagerange of influence and is located between 3 and 10 μm, preferably 5 μm.

In a first variant, the instrument 1 works with a spacing a of the laserfocuses 7, i. e. of the centers of interaction, which is below a maximumvalue amax=10 μm. From this value, the graph for the threshold value Mdrops considerably towards smaller spacings a, making it possible towork with a clearly reduced fluence F.

In a second variant, an upper limit Fmax is employed for the fluence F.The value for this is 5 J/cm².

In a combination of the first and second variants, both a≦amax andF≦Fmax apply. The spacings of the centers of interaction as well as thefluence of the laser pulses are located within the region composed ofpartial areas 18.1 and 18.2 which are yet to be explained. Since thelaser surgical instrument 1, in both variants per se as well as in thecombination of these two variants, respectively generates opticalbreakthroughs in the material, e.g. the cornea 5, the fluence F is, ofcourse, always above the threshold value M, because each laser pulsesecurely generates an optical breakthrough 8 only above said thresholdvalue.

A third variant modifies the second variant such that the fluence F ofeach laser pulse only exceeds the threshold value M at the most by anexcessive energy of between 3 and 3.5 J/cm². The fluence F is then keptbelow the dotted line of FIG. 6 which separates the areas 18.1 and 18.2from each other. Of course, the third variant can also be combined withthe first variant, so that the fluence F and the spacing a are locatedin the hatched area 18.2.

In a different embodiment, the laser surgical instrument 1 works withlaser pulses of which not every single one securely generates an opticalbreakthrough 8. However, in order to achieve material separation inspite of this, the centers of interaction are sequentially arranged at aspacing a which is smaller than the diameter d of the laser focus, i.e.smaller than the size of the zones of interaction. This mode ofoperation is shown in more detail in FIGS. 10-12.

FIG. 10 shows a one-dimensional example of the arrangement of thecenters of interaction Z corresponding to the position of the(theoretical) focal point. Each interaction is generated by a laserpulse, with the focus 7 being diffraction-limited, for example, andhaving the diameter d of 3 μm, for example, as assumed in FIG. 7. Thecenters of interaction, i.e. the center of the focused laser radiation,are then displaced such that adjacently covered zones of interaction 20,21, 23 and 24 respectively overlap with their immediate neighbors. Thus,there are overlapping regions 25, 26, 27, which are each covered by twozones of interaction. The energy introduced into a zone of interactionis below the threshold value M, so that each of the zones of interaction20-24 per se does not securely cause an optical breakthrough. However,due to said overlapping a material-separating effect is still achieved.Thus, it is essential for this mode of operation that the distancebetween the coordinates of the centers of interaction is smaller thanthe extent d of the zones of interaction. FIG. 10 clearly shows that thedistance between the individual coordinates X1, X2, X3 and X4corresponds to approximately half the diameter d of the zones ofinteraction 20-24, which results in a simple overlap.

FIG. 11 shows a narrower graduation of the zones of interaction,ultimately resulting in a four-fold overlap of the zones of interaction.This allows a further reduction of the fluence F.

FIG. 12 illustrates that the representations of FIGS. 10 and 11 are onlyone-dimensional, i.e. considering only the x coordinate, for the sake ofsimplicity. If the zones of interaction overlapping each other in the xdirection are displaced in the y direction, further overlaps will beachieved, so that in spite of the actually just one overlap in the xdirection a three- or five-fold overlap of zones of interaction isachieved in the y direction, depending on the intervals. In this case,the selection of the intervals in the x direction or in the y direction,respectively, allows any desired factors of overlap (2, 3, 4, 5, 6, 7, .. . ).

As a result, the instrument 1 works in the operating range 19, which ischaracterized in that the distance between two subsequent centers ofinteraction is smaller than the extent of the zones of interaction orthan the size of the focus spot and in that the fluence F is below thethreshold value M required to generate optical breakthroughs.

In practice, a spacing of the laser focuses or of the centers ofinteraction, respectively, of approximately 3-5 μm has turned out to bewell-suited for generating high-quality cuts with as little pulse energyas possible and requiring a limited amount of time.

In a laser surgical instrument 1 which produces very fine cuts, e.g. ifthe above-described fluence values are used for the laser pulses, thecut is not visible even immediately upon being produced, either becauseplasma bubbles or gas bubbles appear, having a smaller size and ashorter life than during operation outside the region 18, or because nobubbles form at all (during operation in the region 19). This may makeit more difficult to prepare the isolated cut, e.g. by means of aspatula. A manual procedure used in many applications and whereinresidual bridges which have not yet been fully separated are pierced bya spatula or other tools can become very difficult in case of suchsmooth cut.

In order to avoid this, the control device 17 of the laser surgicalinstrument 1 carries out the division of the cut shown in FIGS. 8 and 9,for example. The cut surface is divided into partial cut surfaces havingdifferent degrees of fineness. These partial cut surfaces are cut withdifferent smoothness so that regions form in which the cut surface hasbetter optical visibility than in other regions.

FIG. 8 shows a top view of the cornea 5 of the patient's eye 1, and FIG.9 shows a sectional view along line A1-A1 of FIG. 8. As can be seen, thecut surface 9 is adapted to isolate the partial volume T, as alreadyschematically indicated in FIG. 3. The cut surface 9 then consists of ananterior portion F and a posterior portion L. The anterior portion F isguided up to a peripheral opening S via a laterally opening cut 16leading up to the corneal surface. Thus, after forming the cut surface 9the portions F, L, 16 and S of the lens-shaped partial volume T arelocated in a pocket formed by the peripheral opening S.

In order that a surgeon may feel this pocket with a spatula or othersurgical instrument so as to sever possible bridges of tissue betweenthe lens-shaped partial volume T and the rest of the cornea 5, theanterior portion F as well as the posterior portion L are respectivelydivided into two partial regions. A core region F1 or L1, which issubstantially circular, is respectively surrounded by an annularperipheral region F2 or L2. In the core region located near the opticalaxis of vision, a small size of plasma bubble, i.e. a fine line ofcutting, is worked with. This may be effected, for example, by operationin the regions 18 or 19 of FIGS. 6 and 7, respectively. In contrastthereto, a comparatively coarser cut is produced in the (annular)peripheral regions L2 and F2, for example by deliberately workingoutside the regions 18 or 19, so that relatively large plasma bubblesform. Thus, in these peripheral regions, the cut surface is a lotrougher and easier to recognize by the surgeon.

The diameters of the central regions F1 and L1 are preferably greaterthan the pupil diameter P of the treated eye. Thus, the peripheralregions F2 and L1, where a rougher cut was employed, are located outsidethe region of the cornea 5 used for optical perception and accordinglydo not have a disturbing effect. The purpose of dividing the portions Land F is to simultaneously achieve the aspect of maximum precision ofcutting as well as of good handling due to the visibility of the cut inthe peripheral region as a result of differences in processing.

If plasma bubbles are employed for material separation, the energy ofthe laser pulses is above the threshold value M. As already mentioned,the shape of the bubbles resulting from the absorption of the laserenergy in the tissue is subject to change over time. A first phase inwhich individual bubbles form is followed by a phase of agglomeration inwhich several individual bubbles join to form larger macrobubbles.Finally, dissipation is noted as the last phase in which the gas contentof the macrobubbles is absorbed by the surrounding tissue until thebubbles have finally vanished again completely. Now, macrobubbles havethe adverse property of deforming the surrounding tissue. If a furthercenter of interaction is placed at a certain position in the deformedtissue to form the beginning of a plasma bubble, the position of thecenter of interaction will change and so will the position of the tissueseparation effected thereby as soon as the phase of dissipation begins,in which the bubbles disappear and the deformed tissue relaxes (at leastpartially). Since the macrobubbles form only after a characteristic timeand are not present already upon introducing laser pulse energy, it isenvisaged for one variant of the laser surgical instrument 1 that thetime between application of the laser energy in two regions of thetissue potentially influencing each other be kept sufficiently short soas to be shorter than a characteristic time which is required to formmacrobubbles.

During isolation of the lens-shaped partial volume T, regions of theposterior and anterior portions of the cut surface 9 having an adverseeffect on each other are located in the region of the optical axis ofvision. If the cut is produced in the anterior portion F of the cutsurface 9 only at a time when the previously processed posterior portionL already comprises macrobubbles, the cut surface of the anteriorportion F is located within deformed tissue. The result after relaxationwould be an undesired undulation of the cut surface 9 in the anteriorportion F. Therefore, the laser surgical instrument 1 produces the cutsurface in the anterior portion F and in the posterior portion L withina time interval which is smaller than the characteristic time it takesfor macrobubbles to form. Typically, such time is approximately 5 s.

One way of achieving this consists in dividing the anterior andposterior portions into corresponding partial surfaces and alternatingbetween the partial surfaces of the posterior and anterior portionsduring production of the cut surface so that at least in the centralregion the characteristic time for producing partial surfaces,posteriorly and anteriorly, is not exceeded. A further possibilityconsists in a suitable sequential arrangement of the centers ofinteraction. Thus, for example, first the posterior portion L can be cutin a spiral leading towards the optical axis of vision from the outsideto the inside and directly afterwards the anterior portion F can be cutin a spiral extending outwards from the axis of vision. The generatedinteractions, at least in a core region around the axis of vision, arethen within the time frame given by the characteristic period of time sothat there is no influence on the macrobubbles during processing of theanterior portion.

During division into the partial surfaces which the laser surgicalinstrument 1 effects under the control of the control device 17 it isensured that a posterior region to be worked on is not disturbed by analready processed anterior surface or zone of interaction acting as ascattering center.

The described cut shapes, surface divisions, etc. are effected by thelaser surgical instrument under the control of the control device 17.The control device 17 causes operation of the laser surgical instrument1 by the process features described herein.

As far as embodiments of the laser surgical instruments have beendescribed above, they can be realized alone as well as in combination,depending on the specific realization of the laser surgical instrument1. Instead of being employed in laser surgery, the instrument 1 can alsobe used for non-surgical material processing, for example in theproduction of wave guides or the processing of flexible materials.

1. A device for material processing by laser radiation, said devicecomprising: a source of laser radiation emitting pulsed laser radiationfor interaction with the material; optics focusing the pulsed laserradiation to a center of interaction in the material; a scanning unitshifting positions of the center of interaction within the material,wherein each laser pulse interacts with the material in a zonesurrounding the center of interaction assigned to said laser pulse sothat material is separated in the zones of interaction; and a controlunit that is configured to control the scanning unit and the source oflaser radiation such that a cut surface is produced in the material bysequential arrangement of zones of interaction, wherein the control unitcontrols the source of laser radiation and the scanning unit such thatthe cut surface includes a first cut surface part and second cut surfacepart wherein the second cut surface part surrounds the first cut surfacepart and wherein the control unit sets a parameter influencingcoarseness of the cut surface by controlling the scanning unit and thesource of laser radiation such that the second cut surface part iscoarser than the first cut surface part.
 2. The device as claimed inclaim 1, wherein a fluence of each laser pulse applied to make the firstcut surface part is above a threshold value M, which is given as M=3.3J/cm²−(2.4 J/cm²)/(1+(a/r²)²) wherein the term “a” is the spacingbetween two adjacent centers of interaction and the term “r” is aparameter representing average range of influence and wherein 3 μm≦r≦10μm.
 3. The device as claimed in claim 2, wherein the fluence of eachlaser pulse applied to make the first cut surface part is no more than 3J/cm² above the threshold value M.
 4. The device as claimed in claim 1,wherein the control unit controls the source of laser radiation and thescanning unit such that the second cut surface part is formed byapplication of a laser pulse fluence greater than about 3 J/cm².
 5. Thedevice as claimed in claim 1, wherein the control unit controls thesource of laser radiation and the scanning unit such that the second cutsurface part is formed by application of a laser pulse fluence greaterthan about 5 J/cm².
 6. The device as claimed in claim 1, wherein in thefirst cut surface part the spatial distance “a” of the centers ofinteraction of two subsequent laser pulses is smaller than a size “d” ofthe focus so that sequentially produced zones of interaction overlap inthe material.
 7. The device as claimed in claim 6, wherein a fluence ofeach laser pulse applied to form the first cut surface part is below athreshold value M, above which an optical breakthrough forms in thematerial.
 8. The device as claimed in claim 7, characterized in that thefluence of each laser pulse applied to form the first cut surface partis below the threshold value M, which is given as M=3.3 J/cm²−(2.4J/cm²)/(1+(a/r²)²) wherein “a” is the spacing between two adjacentcenters of interaction and “r” is a parameter-representing average rangeof influence and wherein 3 μm≦r≦10 μm.
 9. The device as claimed in claim1, wherein the control unit controls the scanning unit and the source oflaser radiation such that in the second cut surface part adjacentcenters of interaction are located at a spatial distance “a” greaterthan ten micrometers from each other and in the first cut surface partthe adjacent centers of interaction are located at a spatial distance“a” less than or equal to about ten micrometers from each other.
 10. Amethod of material processing by laser radiation, comprising generatingand focusing pulsed laser radiation at centers of interaction in thematerial; shifting positions of the centers of interaction in thematerial, each laser pulse interacting with the material in a zonesurrounding the center of interaction assigned to said laser pulse, sothat material is separated in the zones of interaction, and a cutsurface is produced in the material by sequential arrangement of zonesof interaction, wherein; forming the cut surface of two cut surfaceparts including a first cut surface part and a second cut surface part,the first cut surface part surrounding the second cut surface part; andcontrolling a parameter of the focused pulsed laser radiationinfluencing coarseness of the cut surface using a controller such thatthe second cut surface is coarser than the first cut surface.
 11. Themethod as claimed in claim 10, wherein a fluence of each laser pulseapplied to make the first cut surface is above a threshold value M givenas M=3.3 J/cm²−(2.4 J/cm²)/(1+(a/r²)²) wherein “a” is the spatialspacing of the centers of interaction and “r” is a parameterrepresenting average range of influence and wherein 3 μm≦r≦10 μm. 12.The method as claimed in claim 11, wherein the fluence of each laserpulse applied to make the first cut surface is no more than 3 J/cm²above the threshold value M.
 13. The method as claimed in claim 10,wherein the second cut surface part is produced by application of alaser pulse fluence greater than about 3 J/cm².
 14. The method asclaimed in claim 10, wherein the second cut surface part is produced byapplication of a laser pulse fluence greater than about 5 J/cm².
 15. Themethod as claimed in claim 10, wherein in the first cut surface adjacentzones of interaction are located at a spatial distance “a” less than orequal to about ten micrometers and in the second cut surface part theadjacent zones of interaction are located at a spatial distance “a”greater than ten micrometers.